"They took the bold step when their abductors moved out to carry out an operation," he said.

Clashes took place between the Islamists and the army late on Friday after an attack by the insurgents in the town of Damboa, where more than 50 of them were killed, the army had said.

Spokesmen for the armed forces or the government could not be reached for comment.

Activists of the Bring Back Our Girls movement meanwhile tried to march on the presidential palace in Abuja on Sunday in another reminder of the fate of more than 200 girls kidnapped in a separate incident in Chibok, Borno state, on April 14, but were asked by security forces to turn back.

"It's 83 days today that the girls have been abducted," activist Aisha Yesufu told the media.

"We have been coming out for 68 days and nobody has really listened to us," Ms Yesufu told reporters after the march.

She said that was why the group decided to take the protest back to the president.

"So that he will know that we are still out there after the 68 days that we have been coming out daily."

Security experts say the overstretched and under-resourced military is incapable of fighting an effective counterinsurgency against the Boko Haram militants, who have killed thousands in their five-year campaign for an independent Islamic state in the north.